Fuel cell power plants which utilize a fuel cell stack for producing electricity from a hydrocarbon fuel are well known in the art. Each cell in the fuel cell stack includes an anode, a cathode, and an electrolyte disposed there between. Both the anode and the cathode utilize various well known catalysts. In order for the hydrocarbon fuel to be useful in the fuel cell stack's operation, it must first be converted to a hydrogen-rich stream. Hydrocarbon fuels which are used by the fuel cell stack pass through a steam reforming process to create a process gas having an increased hydrogen content which is introduced into the fuel cell stack. The resultant process gas contains hydrogen, carbon dioxide, and carbon monoxide. The process gas has about 10% carbon monoxide (on a dry basis) upon exit from the steam reformer.
Since the anode catalyst of a phosphoric acid fuel cell stack can be poisoned by a high level of carbon monoxide, the level of carbon monoxide in the process gas must be reduced prior to flowing the process gas to the fuel cell stack. This is conventionally done by passing the process gas a through a low temperature shift converter prior to flowing the process gas to the fuel cell stack. The shift converter also increases the yield of hydrogen in the process gas.
Shift converters for reducing the carbon monoxide content of process gas are conventional, and typically comprise a chamber having an inlet, a catalyst bed consisting of copper/zinc oxide pellets, a perforated plate or screen to support the catalyst and a gas outlet downstream of the bed. In operation, a low temperature shift converter carries out an exothermic shift conversion reaction represented by the following equation: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 (1)
The reaction (1) between the carbon monoxide and water concurrently reduces the carbon monoxide content and increases the carbon dioxide and hydrogen content of the process gas. The generation of additional hydrogen from this reaction is an advantage to the power plant since hydrogen is consumed at the fuel cell anode to produce power. Since the shift conversion reaction is exothermic, the heat of reaction increases the temperature of the process gas as it passes through the catalyst bed. The shift conversion reaction should take place within a suitable temperature range, preferably between 275.degree. F. and 550.degree. F., so that the shift conversion catalyst bed is most efficient. If the catalyst bed temperature is too low, the reaction proceeds too slowly. If the temperature is too high, the catalyst can be damaged by thermal sintering. Excessively high temperatures also limit the equilibrium conversion resulting in too high an exit carbon monoxide level. In order for the shift conversion reaction to take place within the aforesaid desired temperature range, the shift conversion catalyst bed must be maintained at proper temperatures.
In the prior art, there are actively-cooled and adiabatic shift converters. The former has a means for actively cooling the shift converter while the latter does not. Both types of shift converters must be designed to meet two major operating parameters, which are reducing the carbon monoxide to a minimum level within the effluent and creating an effluent gas having an outlet temperature below the critical temperature, the critical temperature being the exit temperature above which damage to the fuel cell may occur. This effluent or exit temperature is typically about 425.degree. F. Exit temperatures above this level may require an additional process heat exchanger to cool the process gas before entering the fuel cell. It is also desirable to operate the exit end of the shift converter at or near this critical temperature level as this will insure a low carbon monoxide content. Below this temperature level, the reaction rate is slower requiring additional catalyst volume to achieve the same level of carbon monoxide in the effluent.
Desirably, the shift converter should produce an effluent gas stream having a carbon monoxide level of less than 1%. The carbon monoxide levels in the effluent of the actively-cooled, low temperature shift converters are about 0.2% to 0.3%, while they are about 0.3% to 0.5% in the effluent of the adiabatic low temperature shift converters, both of which are acceptable. The selection of an adiabatic or actively-cooled bed may not result in the most efficient use of catalyst resulting in the smallest required catalyst volume. With the adiabatic bed, the inlet temperature must be low enough to prevent the exit temperature from exceeding the critical outlet temperature. The low average temperature in the bed results in low catalyst activity, low reaction rates and the largest required catalyst volume. Higher levels of carbon monoxide in the inlet process gas require a very low inlet temperature which may be too low to initiate and achieve a sufficient reaction rate.
An actively-cooled bed allows for higher process inlet temperatures, higher average bed temperatures and reaction rates, and active cooling maintains an exit temperature for the gas at or below the critical outlet temperature. The required catalyst volume with an actively-cooled system at the same exit temperature and the same exit carbon monoxide levels as the adiabatic bed are smaller than the required catalyst volume for a purely adiabatic bed.
U.S. Pat. No. 3,825,501 granted Jul. 23, 1974 to J. R. Muenger discloses a muli-stage shift converter which includes an initial adiabatic portion followed by an actively cooled isothermal portion, which in turn is followed by an equilibrium-limited actively cooled portion. The coolant in the actively cooled isothermal portion reaches the temperature of the effluent gas stream from the adiabatic portion so as to hold the temperature of the gas stream steady through the isothermal portion. Thus the temperature of the coolant must rise to the temperature of the gas stream in the isothermal portion of the shift converter. The coolant, if a liquid, must be at a very high pressure in order that its temperature can increase; or the coolant may be steam which can operate at lower pressures than a water coolant. The coolant in the equilibrium-limited actively cooled portion is process gas. The process gas coolant will have its temperature rise in the equilibrium-limited portion of the shift converter so that the process gas will be preheated before the process gas enters the adiabatic portion of the shift converter. The use of a coolant which heats up as it cools the process gas requires a large volume coolant loop and may require very high pressures if a liquid coolant is employed.
U.S. Pat. No. 5,464,606 granted Nov. 7, 1995 to R. F. Buswell et al also discloses a shift converter which has an adiabatic portion and an actively cooled portion. The coolant in the actively cooled portion is process gas. The coolant stream process gas is heated as it cools the gas flowing through the catalyst bed.
It would be desirable to have a two stage shift converter which utilizes a coolant in the actively cooled stage which coolant does not substantially increase in temperature in the actively cooled portion of the shift converter. Such a system would allow for a more compact shift converter and one that would not require excessively high coolant pressures.